1. Field of the Invention
The present invention relates to a process for chemically milling an airplane skin or other component, which eliminates pitting of the surfaces during etching.
2. Description of Related Art
Traditionally, mechanical machining techniques were used to manufacture structures for the aircraft and aerospace industries. However, these mechanical techniques prevented the formation of lightweight complex structures. Chemical milling was developed to overcome the problems associated with the mechanical techniques.
In conventional chemical milling procedures, the workpiece is masked to prevent overall damage, the maskant is then removed from certain areas of the workpiece and the workpiece is exposed to an etching solution so that the unexposed areas of the workpiece are dissolved, thus shaping the workpiece. However, during masking bubbles and other defects are formed on the surfaces creating pin holes in the maskant through which the etching solution can attack the surface of the workpiece causing pitting. These pits cause unsatisfactory variations in the workpiece surfaces, resulting in scrap.
U.S. Pat. Nos. 4,113,549 and 4,137,118 disclose a chemical milling method which consists of a plurality of masking, removal and etching steps. Initially, the entire surface is masked, certain areas of masking are removed and the workpiece is etched. Next, the workpiece is remasked and other areas of the masking are removed and the workpiece is etched again. This continues until the desired configuration is formed. In attempt to prevent pitting, the process rotates the workpiece during etching. However, the above process, like other conventional processes, does not provide a protective layer of film to insure that the maskant layer of the workpiece is protected and etching solution does not reach the workpiece surface to form pits therein.
U.S. Pat. No. 4,588,474 discloses a conventional process for chemical etching and various compositions of etches used therein. Like the above references, no means is provided to protect the maskant layer, and ultimately the workpiece from pitting.
U.S. Pat. Nos. 4,662,984 and 4,689,114 disclose the use of applying protective layers on top of resist films. In the process resist films are applied to almost the entire area of the inner and outer surfaces of a metal plate. Next, a protective layer of film is applied on a first surface, while etching solution is sprayed on a second surface. After etching is completed on the second surface, a second protective film layer is applied on the etched second surface and the first protective film is removed from the first surface. Finally, etching solution is sprayed on the unprotected first surface to form the work piece. The process does not contemplate the use of a plastic layer of film disposed on top of a maskant to prevent pitting of a workpiece.
The improved chemical milling process of the present invention produces a pit-free workpiece despite any defects in the maskant layer. Production time and expense can be reduced without the waste of expensive workpieces.
Advantages of the invention are set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.